In terms of global environmental preservation, it has been required to save pulp consumption and accordingly, it has been required to make paper lightweight and increase the waste paper addition amount. However, the paper obtained simply with a decreased amount of pulp added has deteriorated opacity owing to the thinness of the paper, resulting in inferior quality.
Meanwhile, if the mixing ratio of waste paper is increased, the whiteness is deteriorated owing to the remaining ink in the waste paper pulp and the pulp itself is made thin during the recycling process to result in thin paper having a deteriorated opacity.
In order to prevent the decrease in thickness of paper due to it being lightweight, various methods for increasing the bulk and a method for using cross-linked pulp, a method for mixing synthetic fibers, a method for filling gaps among pulp fiber fillers such as an organic matter or the like, and a method for introducing voids have been known; however these methods are improper for recycling of pulp and deteriorate the smoothness of paper.
On the other hand, in order to improve the whiteness and opacity, a method of adding a large amount (e.g. 5 to 20% by weight) of an inorganic filler such as calcium carbonate, kaolin, white carbon or the like has been employed in this field. However, if a large amount of the inorganic filler is simply added, the weight of the paper is increased significantly and therefore, even if the pulp amount is saved and the inorganic filler is increased, the paper cannot be made lightweight. Particularly, in the case when inorganic filler is added to paper pulp, a large amount of the filler is required to be added and it makes the paper more difficult to be lightweight.
Recently, bulking agents for improving the bulky property of paper by adding an agent to the paper manufacturing process have been disclosed (JP-B 2971447, JP-A 2002-115199).
Conventionally, in a paper product production process, a variety of paper manufacturing chemical agents are added for improving the paper's qualities and its productivity, e.g. improving gloss, being water-proof, its sizing property, its toughness property and the like. These paper manufacturing chemical agents have solubility in water in a wide range.
In the case where agents having high solubility in water, they may be added in a form of an aqueous solution before the paper manufacturing process or applied as a solution to the surface of paper after manufacturing or are impregnated in the paper with the solutions to provide desired functions; however in the case where oil agents for the paper manufacturing have low solubility in water, it is difficult to evenly add or apply the agents if they are not dissolved once in solvents and that results in a problem such as recovery of the solvents.
Use of the oil agents for paper manufacturing having low solubility in water for paper manufacturing leads to technical difficulty of the paper manufacturing process and disadvantages in the process. On the other hand, by utilizing the low solubility of the oil agents for paper manufacturing in water, it has been tried to use the agents as paper quality improving agents. (JP-B 2971447, WO-A 98/03730)
EP-A 220941 discloses a granular solid composition containing a ketene dimer and a sealing agent.